TDC with uncontrolled delay lines: calibration approaches and precision improvement methods

The time-to-digital-converter (TDC) using uncontrolled delay lines has a simple structure and finer measurement precision since the delay cells are pure digital gates that operate at maximum speed. For every incoming hit, two "snapshots" of the delay line are taken by the register array with two strobes separated with a known time interval. With two measurements, propagation delays of each cell in the delay line can be calibrated for the operating temperature and voltage. The two measurements can also be averaged to improve the TDC measurement precision. We will discuss various calibration approaches and present test results in this work.